R-T-B system permanent magnets (wherein R represents one or more rare earth elements and T represents Fe or Fe and Co) in each of which the main phase thereof comprises grains composed of an R2T14B type intermetallic compound (wherein referred to as R2T14B grains in the present invention) have been used in various electric devices and machines because the R-T-B system permanent magnets are each excellent in magnetic properties and a main component of each thereof, Nd, is abundant as a natural resource and relatively inexpensive.
Even the R-T-B system permanent magnets having excellent magnetic properties involve some technical problems to be solved. One of such problems is corrosion resistance. More specifically, the R-T-B system permanent magnets are poor in corrosion resistance because their main constituent elements, namely, R and Fe, are elements susceptible to oxidation. Accordingly, an overcoat to prevent corrosion is formed on the magnet surface. For the overcoat, resin coating, chromate film, plating or the like is adopted; among these, particularly, a method of plating a metal coat typified by Ni plating is frequently used because of being excellent in corrosion resistance, abrasion resistance and the like.
The grain boundary phase (also referred to as R-rich phase), one of the phases constituting each of the R-T-B system permanent magnets, is an origin of the corrosion. Consequently, as a measure for improving the corrosion resistance of the R-T-B system permanent magnets, it is a possible approach that in each of the magnets, the content of the R-rich phase is decreased by reducing the amount of R and the crystal structure of the magnet is made finer.
However, reduction of the content of R degrades the magnetic properties. An R-T-B system permanent magnet is generally produced by means of a powder metallurgy method in which a fine alloy powder of a few microns in particle size is compacted and sintered; such an alloy powder contains a considerable amount of chemically extremely active R, and hence the powder undergoes oxidation during the production steps to result in reduction of the amount of R effective in attaining magnetic properties; and thus, it becomes impossible to overlook the degradation of the magnetic properties, in particular, the degradation of the coercive force. Accordingly, among the R-T-B system permanent magnets there are many examples which are set to contain a relatively large amount of R such as 31 wt % or more.
For the above described problems, Patent Document 1 (Japanese Patent No. 3171426) proposes a sintered permanent magnet which is improved in corrosion resistance by having a composition in terms of percentages by weight such that R (R represents one or more rare earth elements): 27.0 to 31.0%, B: 0.5 to 2.0%, N: 0.02 to 0.15%, O: 0.25% or less, C: 0.15% or less, and the balance being Fe; and the coercive force (iHc) thereof is 13.0 kOe or more. Patent Document 2 (Japanese Patent No. 2966342) also proposes a sintered permanent magnet which has a composition in terms of percentages by weight such that R (R represents one or more rare earth elements): 27.0 to 31.0%, B: 0.5 to 2.0%, N: 0.02 to 0.15%, O: 0.25% or less, C: 0.15% or less, and the balance being Fe; and the sum of the areas of the R2Fe14B grains of 10 μm or less in grain size is 80% or more and the sum of the areas of the R2Fe14B grains of 13 μm or more in grain size is 10% or less, in relation to the total area of the main phase.
The proposal of Patent Document 1 is based on the finding that in the R—Fe—B based sintered permanent magnet which has a rare earth content falling within a specified range, and an oxygen content and a carbon content each being equal to a specified value or less, the corrosion resistance thereof is improved and practical, high magnetic properties can also be obtained by setting the nitrogen content thereof to fall within a specified range. The proposal of Patent Document 2 is also based on the finding that the corrosion resistance of the sintered permanent magnet is further improved by further setting the R2Fe14B grain size to be a certain specified value or less.
As described above, the R-T-B system permanent magnets each has an overcoat formed on the surface thereof by electrolytic plating or the like. Accordingly, the corrosion resistance of an R-T-B system permanent magnet should be investigated under the conditions that the overcoat is formed.
Patent Document 3 (Japanese Patent Laid-Open No. 5-226125), Patent Document 4 (Japanese Patent Laid-Open No. 2001-135511) and Patent Document 5 (Japanese Patent Laid-Open No. 2001-210504) present interesting disclosures for the plating of R-T-B system permanent magnets.
When the Ni plating or Ni alloy plating method is applied to the R-T-B system permanent magnet which has a high hydrogen absorptivity and has a property that hydrogen absorptivity thereof embrittles itself, the hydrogen generated during plating is absorbed inside the R-T-B system permanent magnet, so that brittle fracture and plating exfoliation are caused on the plating interface and the corrosion resistance can no longer be maintained. In this connection, Patent Document 3 proposes that by heating an R-T-B system permanent magnet plated with Ni or a Ni alloy under vacuum at temperatures of 600° C. or higher and lower than 800° C., the hydrogen absorbed during plating in the magnet or in the plating layer is expelled, and thus, for example, the diffusion of the hydrogen in the plating layer into the magnet is prevented on the way of a longtime operation to prevent the hydrogen embrittlement of the magnet interface.
Patent Document 4 points out that the squareness of the demagnetization curve is remarkably degraded when, for example, the magnetic properties are evaluated after magnetizing a magnet with a Ni coat formed by electrolytic plating, and the cause of the degradation is the increase of the hydrogen amount contained in the magnet body and the coat after undergoing coating. Accordingly, Patent Document 4 proposes that electroless plating or vapor phase plating is adopted as the means for forming the overcoat, and the hydrogen amount contained in the magnet body and the coat is controlled to be 100 ppm or less.
Patent Document 5 also proposes that the amount of hydrogen contained in the plating coat of the R-T-B system permanent magnet is to be reduced to 100 ppm or less on the basis of the finding that the thermal demagnetization of the R-T-B system permanent magnet is largely varied depending on the amount of the hydrogen contained in the plating coat.
According to Patent Document 3, the heating under vacuum at temperatures of 600° C. or higher and lower than 800° C. reduces the amount of hydrogen, but tends to degrade the magnetic properties and brings about a fear of degrading the plating coat. The degradation of the plating coat causes the degradation of the corrosion resistance, and hence will be incompatible with the primary purpose of the plating coat. Patent Document 4 does not involve as a subject the electrolytic plating leading to the most effective overcoat in the R-T-B system permanent magnet. According to Patent Document 5, it is necessary electrolytic plating be applied with a low current density and a low voltage; this may bring about a fear of considerable degradation of the production efficiency and no account is taken for the corrosion resistance of the overcoat formed by electrolytic plating.
More sever dimensional precision (for example, to a tolerance of 5/100 mm) than hitherto is recently required for R-T-B system permanent magnets as the case may be. It is the dimensions of a magnet with an overcoat that are required to be severely precise. However, needless to say, the dimensions concerned are significantly affected by the dimensions of the magnet body. To this issue, various approaches have been attempted from the dimensional precision of the magnet body and that of the overcoat. As for the magnet body, it is subjected to barrel polishing treatment before plating so as to round the edge portions thereof which otherwise tend to undergo formation of humps of the plating coat; however, there is a problem such that the surface of the magnet body is partially collapsed (detachment of grains) when thereafter undergoing acid etching and plating coat formation, giving a factor to degrade the dimensional precision of the surface, in particular, the edge portions.
With regard to some of the problems described above, as will be described later, the present inventors have found that it is effective to control the amount or the state of the hydrogen contained in the surface layer portion of the R-T-B system permanent magnet. Accordingly, an object of the present invention is to propose a preferable amount and a preferable state of the contained hydrogen for the R-T-B system permanent magnet, in particular, the R-T-B system permanent magnet with an overcoat formed thereon. This proposal may be sorted out into a plurality of embodiments. According to an embodiment, it is an object to improve the corrosion resistance of the R-T-B system permanent magnet with an overcoat formed thereon without degrading the magnetic properties. In another embodiment, it is an object to provide an R-T-B system permanent magnet compatible with the overcoat formation based on electrolytic plating and capable of fully ensuring the corrosion resistance as a primary target of the overcoat formation without substantially degrading the production efficiency. In yet another embodiment, it is an object to provide an R-T-B system permanent magnet having a high dimensional precision by suppressing the partial collapse (detachment of grains) of the surface thereof.